A thruster unit described above is normally mounted in a swimming vessel so that at first a well is formed in the bottom of said vessel. The well is open downwards but the top of the well is provided with a covering plate by which the well is sealed and closed upwards. The upper gear of the thruster unit is placed above the covering plate which is provided with a sealed lead-in through which the vertical steering tube extends from the upper gear to the underwater section of the thruster unit. The well is conventionally dimensioned so that the underwater section of the thruster unit fits into it. Thereby, if necessary, the thruster unit can be lifted so that the underwater section is completely retracted into the well above the bottom of the vessel.
From time to time the thruster unit needs maintenance and repairs and in some cases it has to be replaced by another thruster unit. In prior art cases in which the thruster unit has to be removed for maintenance or repairs the vessel has normally been brought to dry dock in which it has been possible to execute this kind of maintenance and replacement work. Dry docking of a vessel is extremely expensive because the work itself on a dry dock is expensive and the preparations for taking a vessel to a dry dock are time consuming. Out-of-service time of a large vessel may become quite long and that costs money. Therefore, attempts have been made to get this dead time shorter and to make it possible to get the maintenance and repair work done without bringing the vessel to a dry dock.
Publication WO 2011/127987 describes a mounting method of thruster in which method the maintenance and replacement work is carried out as “dry work” so that the thruster unit is hoisted up through a well or a hoisting chamber onto the deck of the vessel or into a dry maintenance space in which necessary maintenance and repairs are to be done.
Publication WO 97/27102 describes method and apparatus for removing a propeller assembly from and for mounting the same in an opening in the bottom of a swimming vessel. According to this publication the vessel is provided with a watertight hoisting chamber extending from the bottom of the vessel to above the waterline. For maintenance the thruster unit is hoisted through the hoisting chamber from the vessel.
Publications WO 2005/100151 and WO 2009/126097 describe methods and arrangements for disassembly/assembly of tunnel thrusters. Therefore these publications are not directly comparable with the present invention. The main idea in both of these documents is that the assembly work is done as underwater work. For the disassembly/assembly a sledge is mounted in the propeller tunnel, by which sledge the thruster unit is removed and replaced. No dry docking of the vessel is needed.